In the rolling of metal workpieces, the workpiece is passed between a pair of rolls of at least one rolling mill stand. It is usual to pass the workpiece either backwards and forwards through the same mill stand to gradually reduce the thickness of the workpiece or through a plurality of stands arranged in tandem. Particularly in the case of rolling metal strip, a plurality of stands in tandem are used and it will be appreciated that the speed of the strip material through the stands increases as the thickness of the material is reduced. Thus, at the last stand, the strip can be moving at a high speed. For efficient, trouble-free rolling, therefore, it is essential that the moving strip material passes between the rolls substantially centrally between the two mill housings of each mill stand. If this does not occur, and the material gradually moves from this central track towards one of the housings, then it can foul against stationary side guides and the like on the mill housings and cause damage both to the strip and to the mill structure.
It is well known for a rolling mill to be provided with a gauge control system which automatically controls the separation of the rolls of a mill stand to take into account the stretch of the housings as they come under load and also variations in temperature and hardness of the workpiece.
In a rolling mill fitted with a hydraulic cylinder in each housing to adjust the roll gap, the required gauge of the material to be rolled is set by positioning the rolls which form the roll gap and the load on each housing, during rolling, is measured by a separate transducer. These signals are modified by shaping and scaling circuits to represent the stretch of each mill housing and are fed back to the position control loop associated with each housing in such a sense as to make the hydraulic cylinders compensate for the mill stretch. By controlling the magnitude of the signals which are fed back to each position control loop, the apparent stiffness of the mill can be increased from natural stiffness with zero stretch signal feedback to infinitely stiff where the stretch signal fully compensates for the actual stretch of the mill.
The rolls of a rolling mill are eccentric to some degree and, while modern rolls are made to high tolerances, there is always some degree of eccentricity between the rolls when they are fitted in their bearings in the mill housings. If the gauge control circuit is operating such that the apparent stiffness of the mill is approaching infinity, then the effect of the roll eccentricity is imprinted on the strip material being rolled and the gauge of the strip varies cyclically along its length. With a softer mill housing, this eccentricity effect is less pronounced.